As shown in FIG. 10, in a conventional image processing apparatus, such as a digital color copying machine, a color original image is read out by an input device 101, such as a scanner, and the readout image data are separated into three color signals: R(Red), G(Green), and B(blue) (which is collectively referred to as RGB signal hereinafter). Then, the resulting RGB signal is outputted to a first color converting device 102.
The first color converting device 102 carries out color converting processing, that is to say, it converts the RGB signal to chromaticity data based on data pre-stored in a first storage device 106 using Equation (1) below. Equation (1) is a polynomial color converting matrix (MTX) having linear terms and non-linear terms. Note that a coefficient (MTX) used in the color converting matrix is pre-stored in the first storage device 106. ##EQU1## where MTX is a coefficient of the matrix, and k.sub.i (i=1,2,3) is a constant number.
Equation (1) is introduced by, for example, a multiple regression model, by which an input RGB color separation signal of a color sample is used as an input and converted into an L* a* b* value, or desirable chromaticity data. The L* a* b* value referred herein is a value representing one of color specification systems in the homogeneous color space adopted by Commission Internatinale de l'Eclairage (CIE) in 1976.
Next, the chromaticity data (L* a* b* value ) from the first color converting device 102 are inputted into a chromaticity data converting device 103, which compresses a color gamut of the input chromaticity data using chromaticity data pre-stored in a second storage device 107. The chromaticity data pre-stored in the second storage device 107 are chromaticity data of a color reproducible region of an image output device 105, such as a printer, and chromaticity data of a color reproducible region of the input device 101, such as a scanner. An example method of compressing a color gamut is disclosed in Japanese Examined Patent Publication No. 101854/1994 (Tokukouhei 6-101854).
Next, the chromaticity data whose color gamut has been compressed by the chromaticity data converting device 103 are inputted into a second color converting device 104. The second color converting device 104 converts the input chromaticity data into a color signal (YMC) which can be outputted from the image output device 105 based on data pre-stored in a third storage device 108.
That is to say, the second color converting device 104 converts the chromaticity data into desirable YMC based on the data pre-stored in the third storage device 108. To be more specific, like the first color converting device 102, the second color converting device 104 finds a polynomial color converting matrix such that yields desirable YMC based on input data, namely, the L* a* b* value of the color sample of the output device 105, and carries out the color converting processing using the color converting matrix thus found. Therefore, the data pre-stored in the third storage 108 are the coefficient (MTX) used in the color converting matrix.
Next, the color signal from the second color converting device 104 is inputted into the image output device 105, which in turn outputs a copy image of the original image based on the input color signal.
Incidentally, in the conventional image processing apparatus, the chromaticity data converting device 103 compresses the color gamut of all the chromaticity data converted by the first color converting device 102 using the chromaticity data of the color reproducible region of the image output device 105 and the chromaticity data of the color reproducible region of the input device 101. For this reason, the conventional image processing apparatus causes three major problems as follow.
(I) When the input chromaticity data of the original image are converted using the chromaticity data of the reproducible regions of both the output device 105 and input device 101, the input chromaticity data are always processed by the same method. Thus, in some cases, the input image data of an original image may not be processed properly to produce a copy image corresponding to the original image.
(II) Even when the chromaticity data of the original image are within the color reproducible region of the image output device 105, once such chromaticity data are inputted into the chromaticity data converting device 103, the chromaticity data converting device 103 compresses the color gamut of the same unnecessarily. Thus, the saturation of the chromaticity data is lowered more than necessary, and the resulting output image loses brightness.
(III) When the chromaticity data of the original image are out of the color reproducible region of the image output device 105, the color gamut of the same may not be compressed within the color reproducible region of the image output device 105. In such a case, the resulting copy image may not correspond to the original image.